Answer
The river is flowing at a speed of 2 km/h
Work Step by Step
Let $v_b$ be the speed of the boat.
Let $v_r$ be the speed of the river.
We can set up two equations of the form $v = \frac{d}{t}$:
$v_b+v_r = \frac{30~km}{3.0~hr} = 10~km/h$
$v_b-v_r = \frac{30~km}{5.0~hr} = 6~km/h$
We can subtract the second equation from the first equation:
$2v_r = 4~km/h$
$v_r = 2~km/h$
The river is flowing at a speed of 2 km/h.
